The present invention relates to thin films of cadmium sulfide and in particular to a chemical spray deposition process employed in conjunction with a thermal ion exchange process for producing cadmium sulfide, abbreviated herein as CdS.
Cadmium sulfide is a well-known photoconductive semiconductor. Its principal use has been in photodetectors and photovoltaic solar cells (CdS/Cu.sub.2 S cells). In thin film form, these devices display relatively high efficiency at potentially low fabrication costs. Conventionally, these films of CdS are constructed by one of two techniques; at the Institute for Energy Conversion (University of Delaware) for example, vacuum evaporation techniques are used to deposit 20-40 micron layers of CdS on metal substrates. A second technique, used commercially by Photon Power, Inc. of El Paso, Texas, spray deposits CdS from a solution containing CdCl.sub.2 with thiourea as a sulfur source. Specific examples of this technique may be referenced in U.S. Pat. Nos. 4,086,101 and 4,095,006. Both of these techniques produce cadmium sulfide, CdS, directly.
In contrast, the present invention teaches a process for chemical spray deposition of a layer of cadmium oxide which is converted to cadmium sulfide by thermal ion exchange. The deposition of the intermediary oxide is thermodynamically more stable then directly depositing the sulfide, resulting in superior film quality. As taught with respect to differing metals and metal oxides, in a co-pending application, U.S. Ser. No. 31,421 filed Apr. 29, 1979 and assigned to the assignee of the present invention, a more precise control of the film composition may be readily obtained by first depositing an intermediary oxide layer.